There are presently in use in department stores passive tags which are typically affixed to costly garments. The tag comprise a receiving antenna and a transmitting antenna which may be the same, and a harmonic generating unit such as a diode coupled therebetween. When the tag is irradiated with radio frequency (RF) signal of frequency F from an interrogating unit, it radiates to the interrogating unit a harmonic signal at a multiple or multiples of frequency F which is received at a receiving circuit of the interrogating unit. The receiving circuit is responsive to the harmonic signal to set off an alarm. Such a system is described in U.S. Pat. No. 3,798,642 and is appropriate where only the presence or absence of the tag need be noted. The harmonic frequency is necessary because the fundamental frequency F will bounce off of things in the area of interest and be radiated to the interrogating unit causing false alarms.
In the aforementioned prior art system the interrogating unit transmits harmonics of the fundamental frequency being produced thereby and the harmonics also tend to bounce off things in the room or the area of interest and return to the receiving circuit. Since the receiving circuit is expecting to receive the first harmonic of the transmitted signal, the bounce signal appears to be a return signal from a tag, a very undesirable effect. To compensate for this problem either or both the transmitter power and receiver sensitivity must be lowered thereby making the system limited as to the physical area over which it will properly operate.
An automotive collision avoidance system is described in U.S. Pat. No. 4,003,049 which uses a tag disclosed in U.S. Pat. No. 3,984,835. That tag is also receptive of an RF signal of frequency X and has built-in means for modulating the received RF signal with a unique code for each tag and means for radiating the modulated signal. Such a tag is not passive in that it requires power to perform the modulation function.